It refers to a tactic for getting people to agree to something. People who agree to an initial request will often still comply when the requester ups the ante. People who receive only the costly request are less likely to comply with it. A two-step compliance strategy in which the influencer secures agreement with a request by understanding its true cost.
The term "low ball" came from the soccer where a ball which is played low does not seem dangerous and so is acceptable. Then, the attacker raise the ball and has a good chance to get a goal.